This invention relates to a method for the conversion of chemical energy to electric energy. More particularly, this invention relates to a method for conversion of chemical energy to electric energy in a fuel cell of the type in which the conversion is accomplished through the reaction of a fuel such as hydrogen gas with an oxidizing agent such as oxygen gas through the medium of a hydrogen ion conductive solid electrolyte. Still more particularly, this invention relates to a method for the adjustment of the fuel and the oxidizing agent for the purpose of enabling the fuel cell to be stably operated continuously for a long period.
The conventional fuel cells using aqueous electrolyte solution have faults such as inconvenience of handling and leakage of liquid. To take their place, therefore, there have been developed fuel cells using solid electrolytes such as, for example, (1) a fuel cell using an oxide ion conductive solid electrode formed of a solid solution such as ZrO.sub.2 -CaO and (2) a fuel cell using a hydrogen ion electroconductive solid electrode formed of a heteropoly acid such as 12-molybdophosphoric acid.
The fuel cell of (1), however, requires a high temperature of about 1000.degree. C. for the solid electrolyte to acquire electrical conductivity suitable for practical applications. The materials used in this fuel cell must be able to withstand the thermal expansion and corrosion at this elevated temperature. The fuel cell, therefore, entails many problems yet to be solved in this respect.
In the fuel cell of (2), the solid electrolyte is a heteropoly acid hydrate. The electrical conductivity of this solid electrolyte, therefore, is determined by the proportion of hydration.
In order for the fuel cell using such a heteropoly acid solid electrolyte to be operated stably for a long period, it is necessary to maintain the heteropoly acid hydrate within the temperature and humidity ranges at which the crystals of the hydrate are stable.
For example, 12-molybdophosphoric acid H.sub.3 Mo.sub.12 PO.sub.40.29H.sub.2 O has been confirmed to be stable in a temperature range of 10.degree. to 80.degree. C. and a humidity range of 70 to 93% RH (relative humidity).
The solid electrolyte remains in contact with an anodic reactor and a cathodic reactor. It is, therefore, necessary that the temperatures and humidities of these reactants should be maintained within the ranges of stable temperature and humidity of the solid electrolyte.
The temperatures of these reactors can be maintained substantially constant when the fuel cell is operated at room temperature, for example. Nevertheless, the control of their relative humidities is no easy matter. When the absolute steam pressure is maintained constant in the neighborhood of room temperature, a change of temperature by 1.degree. C. results in a change of as large as about 5% in relative humidity. The method adopted for regulating the humidities of the anodic reactant and cathodic reactant suitable for the fuel cell, therefore, must be such that their relative humidities are kept unaffected by any change in their temperatures.
As a method for retaining the relative humidities of given gases at constant levels, the inventors formerly proposed a method resorting to successive passage of such gases through saturated aqueous solutions of salts (Japanese Patent Application No. 38478/1979, U.S. Application Ser. No. 128150).
An object of this invention is to provide a method for enabling the fuel cell described in (2) above to be stably operated by adjusting the temperatures and humidities of the anodic reactant and cathodic reactant, i.e. the fuel and oxidizing agent, of the fuel cell by application of the afore-mentioned method involving successive passage of gases through saturated aqueous solutions of salts.